a millstone around one's neck → μύλος ὀνικὸς περὶ τὸν τράχηλον αὐτοῦ, μύλος ὀνικὸς εἰς τὸν τράχηλον αὐτοῦ

[spiros]

a millstone around one's neck → μύλος ὀνικὸς περὶ τὸν τράχηλον αὐτοῦ, μύλος ὀνικὸς εἰς τὸν τράχηλον αὐτοῦ

βραχνάς, βαριά υποχρέωση, βαριά ευθύνη

[billberg23]

μύλος ὀνικὸς:  literally, "a donkey's millstone," i.e. a millstone heavy enough to require being pulled by a donkey in order to roll around in its circle (cf. Matthew 18:6, Mark 9:42).